Optical switches based on directional coupler devices are known in optical communications. They comprise in general two optical waveguides in close proximity. Optical radiation can be coupled out of one waveguide into the other by means of an evanescent field outside the waveguiding regions. This coupling only takes place over a particular coupling length however, which imposes a minimum device length, typically of the order of mm.
An optical switch may be passive. That is, it may not provide optical gain. However, it has been recognised that there are advantages in using an active device, a device which does provide optical gain, in a directional coupler. The use of an active device allows the optical power losses which inevitably occur in an optical communications system to be at least partially offset.
A known class of active device is that of semiconductor injection lasers which can be used to generate or to amplify an optical beam. A semiconductor injection laser structure comprises in general layers of materials on a substrate, and means for injecting electrical current to the layers, which together provide a longitudinal, waveguiding "active region".
A semiconductor junction is provided in the vicinity of the active region. When the injected current is passed through the layers, across the semiconductor junction, electrons and holes (carriers) combine in the active region to generate optical radiation. The optical radiation may be confined to the active region in a first direction by means of the relative refractive indices (index guiding) and band gaps of the layer materials, and in a second, perpendicular direction by means of the current distribution (gain guiding). The current is usually confined to a specific path by the use of a narrow, "stripe" electrical contact and, optionally, by changes in electrical conductivity of the layer materials in the second direction.
Optical confinement in the second direction may also, optionally, be provided by the distribution of refractive index and band gap of the layer materials by means of stepped interfaces between the layers.
The active region can be considered to be a waveguiding path. That is, a path along which radiation will be guided in use of the laser.
The threshold current at which the onset of "lasing" occurs depends on the degree of feedback into the structure, eg by reflections from its end faces, on the (current dependent) gain through stimulated emission as photons pass along the active region, and on other factors. At currents lower than the threshold current, the laser functions as a light emitting diode or as a superluminescent emitter and can be used to amplify an optical signal. At currents higher than the threshold current, the laser can also be used to amplify an optical input signal.
Another known class of active device is the travelling wave (TW) device. These have the same general structure as a laser but negligible feedback. For instance, the degree of feedback from a reflecting face in a TW device will generally be less than 0.1% and a device has been produced with feedback of the order of 0.01%. The characteristics of a TW device can be substantially different from those of a laser.
A laser (i.e. a non-TW device) generally shows feedback of at least 0.1% or 0.5% and commonly shows substantially greater feedback. If the feedback is provided by reflections at end facets the laser is known as a Fabry-Perot laser and, where the facets are uncoated, generally has a degree of feedback of at least 20%.
In the paper "Directional Coupler Switches With Optical Gain", IEEE Journal of Quantum Mechanics, QE-22(5) pp. 595-602 1986, by C. J. Setterlind and L. Thylen, an active directional coupler is described and theoretically analysed, which incorporates a TW device.
The TW structure described comprises a pair of buried rib waveguides arranged in parallel on a substrate. Each waveguide is provided, above the rib, with a narrow, ie stripe, metal contact and the exposed face of the substrate is metallised to provide a third contact. In one embodiment each stripe contact is in two segments, there being a gap halfway along its length. An optical beam may be input to either one of the waveguides.
By varying the relative currents supplied to the stripe contacts, and to the segments thereof, different distributions of optical radiation between the two waveguides can be achieved. In an ideal "cross state" all the light coupled into one waveguide exits from the other waveguide. In an ideal "bar state" all the light exits from the launch waveguide. Departures from these ideal states are referred to as "cross talk". (Cross talk tends to increase the levels of noise and power loss and is therefore undesirable.)
Although a degree of control over the distribution of light is shown, the TW structure suffers from the disadvantage that the extent to which one can achieve successful coupling from one waveguide to the other, with a low degree of cross talk is a function of the length of the structure and the structure described must necessarily be of the order of 2 mm long.
An object of the present invention is to provide an opto-electronic switch of small dimensions which shows useful performance characteristics.
According to the present invention there is provided an opto-electronic directional switch, comprising a semiconductor laser structure, wherein the laser structure comprises contacts for injecting current to provide at least two distinguishable optical waveguiding paths, the paths being neighbouring in a region where there is a discontinuity in each of the contacts associated with those paths, and control means for varying the characteristics of a first of the paths relative to a second of the paths such that the relationship between light output from the two paths can be changed.
It has been found that by using a laser structure of the above type an active switch can be fabricated which has a length which is an order of magnitude shorter than in known active switches, but which retains good operating characteristics. For instance, a good rejection ratio can be achieved for light output from one of the waveguiding paths compared in different operating modes of the switch. The level of cross talk is significantly less dependent on the length of the structure and it is possible to fabricate an active switch in which the level of cross talk is acceptably low for structure lengths which are an order of magnitude shorter than in known active directional switches.
The optical confinement of the waveguiding paths should be relatively tight. For instance at least 50%, and preferably at least 80%, of the optical radiation propagating in a path should be confined to lie under the associated contact. Typically contact widths will be of the order of 5 microns or less in a semiconductor switch according to an embodiment of the present invention. The confinement may be provided by means in addition to gain guiding provided by the contacts. For instance, index guiding may also be provided.
An additional advantage of the laser structure is that, as long as sufficient feedback is provided for instance using end facets of at least 1% reflectivity in a Fabry-Perot laser, switching can occur with a memory characteristic. That is the device can be switched from one switching state to another by the application of a control pulse which does not need to be maintained subsequently.
Either an electrical or an optical control pulse may be used. That is, the control means may comprise means to vary the relative levels of current injected along each of the paths, or to input an optical signal to at least one of the paths.
The configuration of the waveguiding paths where they are neighbouring in the region of the discontinuities, and of the associated contacts at the discontinuities, should be such that optical radiation propagating in one path can couple into the other path at the discontinuities by means of diffraction at the discontinuities. It is thought that it is because diffractive coupling occurs between the paths that the laser structure length can be so much shorter than in known active directional switches. By utilising coupling in such a manner, the design of the characteristics of the laser structure which determine the waveguiding paths can be relatively flexible. In particular, the paths can be made straight and parallel, optical radiation coupling across at the discontinuities. This is advantageous in fabrication technology and in design, for instance, of an array of several paths.
In an embodiment of the present invention, two parallel, neighbouring stripe contacts may be provided, each being divided into two segments by a discontinuity substantially halfway along its length.